Reciprocating Compressor

ABSTRACT

A reciprocating compressor provided with a piston, which is compact and having high sealing performance is provided. A reciprocating compressor includes: a low-pressure compressor unit having a low-pressure piston and a low-pressure cylinder for compressing air with the low-pressure piston reciprocating while oscillating within the low-pressure cylinder; a high-pressure compressor unit having a high-pressure piston and a high-pressure cylinder for further compressing the air compressed in the low-pressure compressor unit with the high-pressure piston reciprocating while oscillating within the high-pressure cylinder; and a motor for driving the low-pressure compressor unit and the high-pressure compressor unit. A maximum tilt angle during oscillation of the low-pressure piston is made larger than a maximum tilt angle during oscillation of the high-pressure piston.

This application claims the priority of Japanese Patent Application No.JP 2010-092732, filed Apr. 14, 2010, the disclosure of which isexpressly incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a reciprocating compressor in whichpistons reciprocate within cylinders.

2. Description of the Related Art

According to an oscillating compressor disclosed in Japanese PatentApplication Laid-Open Publication No. 2007-32532, in a two-stagereciprocating compressor in which air compressed in a low-pressurecompressor unit is compressed in a high-pressure compressor unit, apiston reciprocates while oscillating within a cylinder, therebycompressing air in each of the low-pressure compressor unit and thehigh-pressure compressor unit.

According to a two-stage compressor disclosed in Japanese PatentApplication Laid-Open Publication No. H11-62822, in a two-stagereciprocating compressor in which air compressed in a low-pressurecompressor unit is compressed in a high-pressure compressor unit, apiston with a piston body fixed to a connecting rod is used in thelow-pressure compressor unit, so that the piston reciprocates whileoscillating (tilting) within a cylinder. On the other hand, a pistonwith a piston body oscillating with respect to a leading end of aconnecting rod is used in the high-pressure compressor unit, so that aleading end of the piston reciprocates without tilting within acylinder.

In the oscillating compressor disclosed in Japanese Patent ApplicationLaid-Open Publication No. 2007-32532, the piston is formed withoutconsideration of a tilt angle of the piston within the low-pressure orhigh-pressure cylinder. It is therefore not possible to improve thesealing performance and lifetime of the piston.

In the two-stage compressor disclosed in Japanese Patent ApplicationLaid-Open Publication No. H11-62822, the piston with the piston bodyoscillating with respect to the leading end of the connecting rod isused in the high-pressure compressor unit. This causes an increase inthe axial length of the piston, and therefore it is difficult todownsize the piston.

SUMMARY OF THE INVENTION

Accordingly, in view of the above-described problems, an object of thepresent invention is to provide a reciprocating compressor with pistonsof low- and high-pressure compressor units being compact and having highsealing performance by forming the pistons in consideration of tiltangles of the pistons within cylinders.

In order to address the problems discussed above, according to oneaspect of the present invention, there is provided a reciprocatingcompressor including: a low-pressure compressor unit having alow-pressure piston and a low-pressure cylinder for compressing air withthe low-pressure piston reciprocating while oscillating within thelow-pressure cylinder; a high-pressure compressor unit having ahigh-pressure piston and a high-pressure cylinder for furthercompressing the air compressed in the low-pressure compressor unit withthe high-pressure piston reciprocating while oscillating within thehigh-pressure cylinder; and a motor for driving the low-pressurecompressor unit and the high-pressure compressor unit. A maximum tiltangle during oscillation of the low-pressure piston is made larger thana maximum tilt angle during oscillation of the high-pressure piston.

According to another aspect of the present invention, there is provideda reciprocating compressor including: a motor having a rotating shaft; alow-pressure compressor unit having a low-pressure cylinder and alow-pressure piston for compressing air; and a high-pressure compressorunit having a high-pressure cylinder and a high-pressure piston forfurther compressing the air compressed in the low-pressure compressorunit. The low-pressure piston and the high-pressure piston each include:an eccentric portion for performing eccentric motion with rotation ofthe rotating shaft of the motor; a connecting rod extending from theeccentric portion; and a piston body provided on a leading end of theconnecting rod. When an eccentric amount of the eccentric portion of thelow-pressure piston with respect to the rotating shaft of motor isrepresented by r1 and an eccentric amount of the eccentric portion ofthe high-pressure piston with respect to the rotating shaft of motor isrepresented by r2, the low- and high-pressure pistons are formed in sucha manner that r1>r2 is established.

According to the present invention, it is possible to provide atwo-stage reciprocating compressor provided with a piston, which iscompact and having high sealing performance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become fully understood from the detaileddescription given hereinafter and the accompanying drawings, wherein:

FIG. 1 is a diagram showing a low-pressure compressor unit and ahigh-pressure compressor unit of a compressor according to an embodimentof the present invention;

FIG. 2 is an enlarged view of the high-pressure compressor unitaccording to the embodiment of the present invention;

FIG. 3 is a general view of the reciprocating compressor according tothe embodiment of the present invention; and

FIG. 4 is a diagram showing oscillating motion of a piston according tothe embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

An embodiment of the present invention will be described with referenceto FIGS. 1 to 4.

A compressor according to the embodiment of the present invention willbe described with reference to FIG. 1. The compressor according to thisembodiment has a crankcase 1. A motor 3 having a shaft (a rotatingshaft) 2 is attached to the crankcase 1. A low-pressure piston 4including a connecting rod 4A and a piston body 5, and a high-pressurepiston 8 including a connecting rod 8A and a piston body 11 are attachedto the shaft 2 of the motor 3 through eccentric portions 7 and 13 forconverting a rotational motion to a reciprocating motion. Thelow-pressure piston body 5 is composed of a retainer 5A, and thehigh-pressure piston body 11 is composed of a base 11A and a top 12A.

The low-pressure piston body 5 is provided with a lip ring 6. Thehigh-pressure piston body 11 is provided with a lip ring 9 and a pistonring 10.

The lip ring 6 is provided with a skirt portion (a lip portion) forincreasing a contact area with a cylinder 14. The skirt portion ismounted facing a low-pressure compression chamber, thereby allowing theensuring of the sealing performance between the piston 4 and thecylinder 14.

Next, a description will be given with reference to the enlarged view ofthe high-pressure compressor unit shown in FIG. 2. The lip ring 6, thelip ring 9, and the piston ring 10 are made of resin material havingexcellent resistance to abrasion and self-lubricating property and eachformed in an approximately annular shape. The piston ring 10 issubstantially rectangular in cross section, and has a uniform radialwidth over the almost entire circumference thereof. Also, an abutmentjoint (not shown) is circumferentially formed on the piston ring 10.This abutment joint allows expansion and contraction in diameter whilemaintaining the sealing performance. Additionally, when thehigh-pressure piston 8 is at the top dead center position or the bottomdead center position, the inner diameter of the piston ring 10 in astate brought into contact with an inner peripheral surface of ahigh-pressure cylinder 17 to be described later is larger than thesmallest diameter of a piston ring groove into which the piston ring 10is mounted. Thus, the piston ring 10 is radially movable with respect tothe high-pressure piston 8.

The lip ring 9 is mounted in the direction opposite to the lip ring 6(the skirt portion of the lip ring 9 faces the crankcase), so that thecenters of the lip ring 9 and the piston body 11 are aligned. Also, whenthe cylinder 17 is assembled in the crankcase 1, the lip ring 9 isbrought into contact with an inner wall surface of the cylinder 17 todefine an assembly position of the cylinder 17, so that the centers ofthe cylinder 17 and the piston body 11 are aligned. Thus, centering ofthe piston ring 10 mounted on the piston body 11, and the cylinder 17can be performed. Further, the lip ring 9 can prevent contact betweenthe piston body 11 and the cylinder 17 when the piston ring 10 becomesworn, and therefore the lifetime of the piston body 11 and the cylinder17 can be improved. In addition, the lip ring 9 is interposed betweenthe base 11A and the connecting rod 8A. Thus, compression heat generatedin the cylinder 17 can be prevented from being transmitted from thepiston body 11 to the connecting rod 8A, so that the temperature of alarge end can be lowered. It is therefore possible to improve the livesof bearings provided on outer peripheries of the eccentric portions 7and 13.

The lip ring 6 is mounted on the low-pressure piston 4, thereby allowingreduction in manufacturing cost. Also, the lip ring 9 and the pistonring 10 are mounted on the high-pressure piston 8, thereby allowingimprovement in assembly performance, sealing performance, and resistanceto abrasion.

The crankcase 1 is mounted with the low-pressure cylinder 14, an airvalve 15, a cylinder head 16, the high-pressure cylinder 17, an airvalve 18, and a cylinder head 19. With the low-pressure piston 4including the low-pressure connecting rod 4A, and the high-pressurepiston 8 including the high-pressure connecting rod 8A, compressionchambers are formed within the respective cylinders.

In the compressor according to the embodiment of the present invention,when the motor 3 is driven to rotate the shaft 2, the eccentric portions7 and 13 allow the low- and high-pressure pistons 4 and 8 to reciprocatewithin the cylinders 14 and 17, respectively, thereby driving thelow-pressure compressor unit including the low-pressure piston 4 andcylinder 14, and the high-pressure compressor unit including thehigh-pressure piston 8 and cylinder 17. Also, the compressor accordingto the embodiment of the present invention has a two-stage compressionstructure in which the low-pressure compressor unit sucks air into thecylinder 14 through the cylinder head 16 and the air valve 15 from theatmospheric pressure and compresses the sucked air to discharge thecompressed air to the high-pressure compressor unit through a pipe (notshown), and then the high-pressure compressor unit sucks the compressedair discharged from the low-pressure compressor unit and furthercompresses the air to discharge the air to a storage tank. At this time,the low- and high-pressure compressor units and the pipe is air-cooledby the rotation of a fan 20 mounted coaxially with the shaft 2.

In general, in the case of air compressors which perform single-stagecompression, air is sucked from the atmospheric pressure and compressedto the maximum pressure by a piston to be discharged. However, due tocompression heat generated at the time of compression, dischargeefficiency decreases, or, noise, vibration and the like due to torquefluctuations of a motor are generated. Therefore, the motor needs tohave the capability for providing a high power output, resulting in anincrease in size. Also, under high pressure, a pressure ratio (dischargeabsolute pressure/suction absolute pressure) increases, thereby raisingthe discharge temperature and deteriorating the suction efficiency.Therefore, it becomes difficult to ensure the discharge air quantity.Furthermore, the rise of the discharge temperature might cause anincrease in distortion and leakage of an air valve. Also, in compressorswhich perform intermittent running, moisture in the suction air iscondensed, thereby increasing the occurrence of drain, which might causea failure.

Therefore, for example, in the compressor disclosed in Japanese PatentApplication Laid-Open Publication No. H11-62822, the above-describedproblems are avoided by adopting the following two-stage compressionstructure. For example, in order to raise the pressure to about 1 MPa ormore, on the low-pressure side, air is sucked from the atmosphericpressure to be subjected to pressure rising and temporarily intermediatecooling, and then on the high-pressure side, the air subjected topressure rising and intermediate cooling by the low-pressure side issucked and further subjected to pressure rising to be discharged. Inparticular, portable small compressors adopt the two-stage compressionstructure in view of low vibration, low noise, miniaturization(downsizing of a motor, weight reduction of a compressor unit).

In the two-stage compression, the respective pressure ratios on the low-and high-pressure sides are reduced, relative to the case of thesingle-stage compression, thereby allowing improvement in efficiency,reduction in heat generation, and reduction in performance deteriorationcaused by the heat generation described above. Furthermore, torquefluctuations of the motor can be reduced and therefore low vibration andlow noise can be realized.

Next, the design of a typical two-stage compressor will be described.

The two-stage compressor disclosed in Japanese Patent ApplicationLaid-Open Publication No. H11-62822 adopts a reciprocating pistonstructure in which a connecting rod is connected to a piston through aneedle bearing on the high-pressure side. In the embodiment of thepresent invention, on the other hand, such needle bearing is removed,and a rocking piston mechanism with the connecting rod and the pistonbody integrally formed is employed, thereby realizing durabilityimprovement by reducing movable portions, weight reduction and lownoise, and reduction in cost by reducing the number of components.

Also, the oscillating compressor disclosed in Japanese PatentApplication Laid-Open Publication No. 2007-32532 includes the pistonsformed without relatively considering the angles at which the pistonsare tilted within the low- and high-pressure cylinders. Consequently, inthe low-pressure compressor unit, the piston tilt angle is minimized, inspite of the fact that, even when the piston tilt angle is made largerthan that of the high-pressure side, its influence on the sealingperformance and abrasion property is small. For this reason, thedownsizing and weight reduction of the piston cannot be sufficientlyachieved. Also, in the high-pressure compressor unit, the piston tiltangle is not designed to be made sufficiently small, in spite of thefact that, when the piston tilt angle is made too large, its influenceon the sealing performance and abrasion property increases. For thisreason, the sealing performance and lifetime of the piston cannot beachieved.

In view of the foregoing, in the embodiment of the present invention,the low-pressure piston 4 and the high-pressure piston 8 include therocking piston mechanism in which: the connecting rods 4A and 8A and thepiston bodies 5 and 11 are integrally formed; the piston bodies 5 and 11are tilted as the connecting rods 4A and 8A are tilted; and the pistonbodies 5 and 11 reciprocate while oscillating within the cylinders 14and 17. Also, the compressor according to this embodiment is designed inconsideration of the angles at which the low-pressure piston 4 and thehigh-pressure piston 8 are tilted within the cylinders 14 and 17,respectively.

Hereinafter, the design of the two-stage compressor according to theembodiment of the present invention will be described in detail.

The motor powers, that is, the shaft powers (the workloads w) of thelow- and high-pressure sides increase with an increase in the respectivepressure ratios of suction pressure to discharge pressure. The shaftpower of the two-stage compressor depends on the sum of the shaft powersof the low- and high-pressure sides. Also, the shaft power decreases asthe pressure ratios on the low- and high-pressure sides decrease. Thatis to say, when the pressure ratios on the low- and high-pressure sidesbecome equal, the shaft power becomes minimum. In view of the foregoing,in the embodiment of the present invention, various sizes (such as borediameter and stroke) of the low- and high-pressure pistons 4 and 8 aredesigned based on the shaft power in consideration of the maximumpressure of the compressor.

Here, the motor shaft powers of the low- and high-pressure sides arecalculated. Required shaft power Ls and theoretical adiabaticaerodynamic force Lad are expressed by the following equations (1) and(2):

$\begin{matrix}{{Ls} = \frac{Lad}{\eta {ad}}} & (1)\end{matrix}$

where Ls represents the required shaft power; and η_(ad) represents theoverall adiabatic efficiency.

$\begin{matrix}{{Lad} - {\frac{k}{k - 1} \times \frac{P_{s}Q_{s}}{0.060} \times \left\{ {\left\{ \frac{Pd}{Ps} \right\}^{\frac{k - 1}{k}} - 1} \right\}}} & (2)\end{matrix}$

where Lad represents the theoretical adiabatic aerodynamic force; Qsrepresents the actual air volume in suction condition; Ps represents thesuction absolute pressure; Pd represents the discharge absolutepressure; and κ represents the specific heat ratio.

Here, the actual air volume Qs shown in the equation (2) is determinedby the parameters of the members composing the compressor and theefficiency of the compressor, as expressed by the following equation(3):

$\begin{matrix}{{Qs} = {\frac{\pi}{4} \times D^{2} \times S \times N \times \eta \; v}} & (3)\end{matrix}$

where D represents the bore diameter; S represents the stroke; Nrepresents the rotational speed; and ηv represents the volumetricefficiency.

As described above, the motor power (the total shaft power) decreases asthe pressure ratios on the low- and high-pressure sides in the two-stagecompression and the difference in shaft power therebetween decrease. Inthe embodiment of the present invention, therefore, in order to minimizethe motor power, assuming that the pressure ratios on the low- andhigh-pressure sides in the two-stage compression are equal, the varioussizes of the low- and high-pressure pistons 4 and 8 are calculated asexpressed by the following equations (4) and (5):

L=Ls1+Ls2  (4)

where L represents the total shaft power; Ls1 represents thelow-pressure required shaft power; and Ls2 represents the high-pressurerequired shaft power.

Pm/P1=P2/Pm  (5)

where Pm represents the intermediate absolute pressure; P1 representsthe low-pressure suction absolute pressure; and P2 represents thehigh-pressure discharge absolute pressure.

On the basis of the equations (1) and (2), Ls1 and Ls2 are expressed asfollows:

$\begin{matrix}{{{Ls}\; 1} = {\frac{1}{\eta \; {ad}}\frac{k}{k - 1} \times \frac{P\; 1{Qs}\; 1}{0.060} \times \left\{ {\left\{ \frac{Pm}{P\; 1} \right\}^{\frac{k - 1}{k}} - 1} \right\}}} & (6)\end{matrix}$

where Qs1 represents the actual air volume in suction condition on thelow-pressure side.

$\begin{matrix}{{{Ls}\; 2} = {\frac{1}{\eta \; {ad}}\frac{k}{k - 1} \times \frac{{PmQs}\; 2}{0.060} \times \left\{ {\left\{ \frac{P\; 2}{Pm} \right\}^{\frac{k - 1}{k}} - 1} \right\}}} & (7)\end{matrix}$

where Qs2 represents the actual air volume in suction condition on thehigh-pressure side.

Here, in consideration of the equation (5), using the constant K, theequations (6) and (7) are transformed as follows:

Ls1=P1·Qs1×K  (8)

Ls2=Pm·Qs2×K  (9)

The equation (3) is substituted into the equations (8) and (9) as:

Ls1=D1² ×S1×P1×k  (10)

Ls2=D2² ×S2×Pm×k  (11)

where D1 represents the low-pressure bore diameter; D2 represents thehigh-pressure bore diameter; S1 represents the low-pressure stroke; andS2 represents the high-pressure stroke.

The motor shaft power becomes minimum when the low- and high-pressuresides are equal in pressure ratio and shaft power. Therefore, thefollowing equations are obtained by the equations (10) and (11):

$\begin{matrix}{{D\; 1^{2} \times S\; 1 \times P\; 1} = {D\; 2^{2} \times S\; 2 \times {Pm}}} & (12) \\{\frac{Pm}{P\; 1} = {{\frac{P\; 2}{Pm}\mspace{31mu} {Pm}} = \sqrt{\frac{P\; 2}{P\; 1}}}} & (13)\end{matrix}$

Based on the above, in the embodiment of the present invention,respective bore diameters and strokes of the low-pressure piston 4 andthe high-pressure piston 8 are determined by the equations (12) and(13).

Note that, in the equations (12) and (13), P2 must be made sufficientlylarge relative to P1 to obtain compressed air at high pressure. That isto say, in the equation (12), Pm must be made sufficiently largerelative to P1. In this case, D1 must be made sufficiently largerelative to D2, or S1 must be made sufficiently large relative to S2 tosatisfy the equation (12) (to at least approximate the left- andright-hand values).

Here, the relationship between the low-pressure bore diameter D1 and thehigh-pressure bore diameter D2 will be described.

Firstly, the low-pressure bore diameter D1 and the high-pressure borediameter D2 are preferably set to satisfy D1>D2. This is because, in thecase of the two-stage compression, the pressure in the high-pressurecompression chamber is higher than that in the low-pressure compressionchamber, and therefore the area of the high-pressure piston 8 is reducedto reduce a load thereon so as to reduce a load F of the piston 8applied in the longitudinal direction of the connecting rod 8A from atop surface of the piston 8 and downsize the bearing provided on theouter periphery of the eccentric portion 13 on the center side of theconnecting rod 8A.

Meanwhile, as will be described later, in the embodiment of the presentinvention, in consideration of the center-of-gravity balance of theproduct, it is necessary to prevent the high-pressure bore diameter D2from being extremely small relative to the low-pressure bore diameterD1.

The center-of-gravity balance of the product is important, especially inportable air compressors. As shown in FIG. 3, the portable aircompressor is mounted with a compressor body 21 including the low- andhigh-pressure compressor units described in FIG. 1, and the motor 3, forexample, on a pair of air tanks 22 (roughly in the center thereof).Also, various auxiliary components, in particular, a reducing valve 23(26), a pressure gauge 24 (27), and an air outlet coupler 25 (28), whichare large in mass, are mounted symmetrically with respect to thecompressor body 21, thereby performing layout in consideration of thecenter-of-gravity balance of the product.

With regard to the compressor body 21 with the largest mass thereamong,the center-of-gravity balance of the compressor body itself is alsoimportant.

As described above, the low- and high-pressure bore diameters D1 and D2and the low- and high-pressure strokes S1 and S2 are determined so thatthe low- and high-pressure sides are made equal in shaft power andpressure ratio. The two-stage compression structure according to theembodiment of the present invention as shown in FIG. 1 includes theopposed two cylinders, and therefore the center-of-gravity balance ofthe compressor body 21 is greatly influenced by the lengths and sizes ofthe low- and high-pressure cylinders 14 and 17, air valves 15 and 18,cylinder heads 16 and 19 protruding from the crankcase 1. As describedabove, it is a common practice to make the high-pressure bore diameterD2 smaller than the low-pressure bore diameter D1 to reduce thehigh-pressure piston load F. However, an extreme difference between thebore diameters D1 and D2 causes differences in size, not only betweenthe connecting rods 4A and 8A but also between the cylinders 14 and 17forming the respective compression chambers, the air valves 15 and 18,and the cylinder heads 16 and 19, thereby losing the lateral balance ofthe compressor body 21. As a result, the center-of-gravity balance ofthe overall product is deteriorated. This also occurs in the case wherethere is a difference in lengths l (the lengths from the centers of theeccentric portions 7 and 13 to the leading ends, i.e. top surfaces, ofthe piston bodies 5 and 11) of the low- and high-pressure connectingrods 4A and 8A. In this case, in addition to the deterioration in thecenter-of-gravity balance, the air cooled by the cooling fan 20 mountedon the compressor body 21 is less likely to reach the larger one in thelength from the compressor body 21 to the cylinder head 16 (19), therebyproducing an increase in temperature. As a result, performancedeterioration and lifetime shortening might be caused. Also, when thecompressor body 21 is mounted on the air tanks 22, the cylinder head ofthe larger one in length protrudes from between the air tanks 22. As aresult, a disadvantage of an increase in dimension of the product mightoccur.

Therefore, in the light of the balance of the product, making an extremedifference in bore diameter D and connecting rod length l between thelow- and high-pressure sides is preferably avoided.

Referring to FIG. 3, the compressor body 21 is mounted in such a mannerthat the axial direction of the shaft 2 and the longitudinal directionof the air tanks 22 are perpendicular to each other, thereby satisfyingboth miniaturization and weight balance. However, the present inventionis not limited thereto, and the compressor body 21 may be mounted insuch a manner that the axial direction of the shaft 2 and thelongitudinal direction of the air tanks 22 face in the same direction.In this case, by locating the shaft 2 in between the two air tanks 22,the center-of-gravity balance can be ensured. Furthermore, the size isdetermined so that the cylinder head is prevented from protruding frombetween the air tanks 22, thereby allowing realization ofminiaturization.

In view of the above, it is necessary to make the high-pressure borediameter D2 smaller than the low-pressure bore diameter D1 to reduce thepiston load on the high-pressure side, while it is necessary to preventD1 from being extremely large relative to D2 to keep balance. In theembodiment of the present invention, therefore, D1, D2, S1, and S2 aredetermined in consideration of the foregoing.

In the equation (12), when S1<S2 is set without making a largedifference between D1 and D2, Pm is not made sufficiently large relativeto P1. Thus, in the equation (13), P2 is not made sufficiently largerelative to P1. More specifically, no compressed air at high pressure isobtainable. In the embodiment of the present invention, therefore, S1>S2is set, thereby making the low- and high-pressure shaft powers subequaland preventing the center-of-gravity balance between the high- andlow-pressure sides from being largely lost, so that compressed air athigh pressure is obtainable.

For example, in order to prevent the center-of-gravity balance frombeing largely lost, D1 is preferably set twice or less as large as D2.In this case, if S1<S2 is set, Pm becomes four times or less as large asP1, and thus compressed air at sufficiently high pressure cannot beobtained. Therefore, in order to obtain compressed air at sufficientlyhigh pressure, S1 must be designed large relative to S2, and S1>S2 mustbe set.

Next, the oscillating motion of the piston 4 (8) and the connecting rod4A (8A) in the rocking piston mechanism will be described with referenceto FIG. 4.

As shown in FIG. 4, while the piston 4 (8) and the connecting rod 4A(8A) move toward the top dead center position and the bottom dead centerposition during the suction and discharge processes, the connecting rod4A (8A) becomes oblique with respect to a central shaft 20 of thecylinder 14 (17) by the eccentricity of the eccentric portion 7 (13).

The maximum tilt angle of the piston 4 (8) with respect to the cylinder14 (17) during oscillation will be described. During oscillation of thepiston 4 (8) within the cylinder 14 (17), when the maximum angle atwhich the longitudinal axis of the connecting rod 4A (8A) is tilted withrespect to the cylinder central shaft 20, or the maximum angle at whicha virtual plane normal to the cylinder central shaft 20 and the topsurface of the piston attached to a connecting rod upper portion aretilted, is set as a tilt angle θ, the tilt angle θ is determined by, thelength l of the connecting rod 4A (8A), that is, the length from thecenter of the eccentric portion 7 (13) to the leading end (top surface)of the piston body 5 (11), and the eccentric amount r of the eccentricportion 7 (13) with respect to the shaft 2 of the motor 3, as expressedby the following equation (14):

$\begin{matrix}{\theta = {\tan^{- 1}\left( \frac{r}{1} \right)}} & (14)\end{matrix}$

where l represents the connecting rod length; and r represents theeccentric amount (=stroke S/2).

Since S1>S2 is set as described above, when the low-pressure eccentricamount is represented by r1 and the high-pressure eccentric amount isrepresented by r2, the relationship therebetween is r1>r2. Also, sincethe low-pressure connecting rod length l1 and the high-pressureconnecting rod length l2 are made subequal to stabilize the center ofgravity of the compressor including the low- and high-pressurecompressor units, the relationship of r1/l1>r2/l2 is established. Thus,on the basis of the equation (14), when the low-pressure maximum tiltangle is represented by θ1 and the high-pressure maximum tilt angle isrepresented by θ2, the relationship of θ1>θ2 is established.

Therefore, regarding the center-of-gravity balance of the compressorbody as described above, the tilt angle θ is minimized while consideringdifferences in the bore diameter D and the connecting rod length lbetween the low- and high-pressure sides, and the low- and high-pressurebore diameters D and strokes S (the stroke S equals two multiplied bythe eccentric amount r), and the connecting rod lengths l are determinedby the foregoing equations while preventing the low-pressure maximumtilt angle θ1 from becoming smaller than the high-pressure maximum tiltangle θ2. Thus, occurrence of the performance deterioration due toleakage of the compressed air is avoided, in particular, the performancedeterioration on the high-pressure side can be prevented. Further, byequalizing the center-of-gravity balance of the compressor body,compressed air at high pressure can be obtained while preventing thedeterioration in the center-of-gravity balance of the product andunequal cooling.

Hereinafter, further advantages obtained by designing the compressor soas to prevent the maximum tilt angle θ2 of the high-pressure piston 8with respect to the high-pressure cylinder 17 from becoming larger thanthe maximum tilt angle θ1 of the low-pressure piston 4 with respect tothe low-pressure cylinder 14 will be described.

In the embodiment of the present invention, the piston body 5 of thelow-pressure piston 4 is mounted with the lip ring 6 that is flexibleand has high followability to a change in the gap between the cylinder14 and the lip ring 6, thereby reducing the tendency to cause theperformance deterioration due to leakage of compressed air from a gapformed in an oscillating direction of the piston 4. On the other hand,the piston body 11 of the high-pressure piston 8 is mounted with thepiston ring 10 requiring stiffness under high pressure and temperature.The piston ring 10 is poor in followability to a change in the gaprelative to the lip ring 6, and thus the performance deterioration dueto leakage of compressed air is likely to be caused. It is thereforenecessary to prevent the high-pressure side from being affected byleakage of compressed air, in consideration of the angle at which thepiston 8 is tilted with respect to the cylinder 17.

While the connecting rod 4A (8A) moves toward the top dead centerposition and the bottom dead center position during the suction anddischarge processes, the connecting rod 4A (8A) becomes oblique withrespect to the central shaft 20 of the cylinder 14 (17) by theeccentricity of the eccentric portion 7 (13). At this time, the contactsurface between the lip ring 6 (the piston ring 10) and the cylinder 14(17) is formed in an elliptical shape (as viewed from the upper side ofthe cylinder central shaft) with the oscillating direction (thehorizontal direction in FIG. 4) corresponding to the long axis. Thus, agap between a side 6A (9A) in the oscillating direction of the lip ring6 (the piston ring 10) and the cylinder 14 (17) is likely to be formed.In particular, during the compression process in which the piston 4 (8)moves toward the top dead center position, the performance deteriorationis likely to be caused due to leakage of compressed air from the gap.

Therefore, in the embodiment of the present invention, as describedabove, the compressor is designed so as to prevent the high-pressuremaximum tilt angle θ2 from becoming larger than the low-pressure maximumtilt angle θ1. Therefore, a maximum gap T2 formed between thehigh-pressure piston 8 and the cylinder 17 can be prevented frombecoming larger than a maximum gap T1 formed between the low-pressurepiston 4 and the cylinder 14. This allows prevention of occurrence ofthe performance deterioration due to leakage of compressed air, inparticular, on the high-pressure compressor unit.

It should be noted that, in the embodiment of the present invention, thepiston ring 10 having stiffness higher than the lip ring is mounted onthe high-pressure piston body 11, thereby also reducing the tendency tothe performance deterioration due to abrasion.

Also, although the embodiment of the present invention has beendescribed by using the case where the lip ring 6 is provided on thelow-pressure piston body 5, a piston ring may be provided in place ofthe lip ring, in the same manner as the high pressure side. In thiscase, followability to the gap is reduced on the low-pressure compressorunit, however, since pressure in the compression chamber of thelow-pressure compressor unit is lower than that of the high-pressurecompressor unit, the low-pressure piston body 5 with the piston ring maybe adopted by taking a measure such as reducing the thickness of thepiston ring to thereby increase the followability. When the low-pressurepiston body 5 is provided with the piston ring, the performancedeterioration due to abrasion of the piston 4 can be also prevented onthe low-pressure compressor unit.

Furthermore, although the embodiment of the present invention adoptingboth the lip ring 9 and the piston ring 10 for use in the high-pressurepiston body 11 has been described, if the performance deterioration dueto abrasion becomes no problem, in place of the piston ring 10, a singlelip ring may be provided facing the high-pressure compression chamber,in the same manner as the low-pressure piston body 5. Alternatively,such lip ring may be used together with the lip ring 9. In this case,further reduction in weight and cost of the high-pressure piston 8 canbe realized.

Moreover, regarding the pressure in the compression chamber, the gasload F to be applied toward the connecting rod central axis from thepiston top surface (in the case where the piston body 5 (11) is theupper side and the connecting rod 4A (8A) is the lower side), and ahorizontal component f of the gas load F occur, thereby pressing the lipring 6 (9) against the cylinder 14 (17). Thus, the surface abrasion ofthe lip ring 6 (9) or the cylinder 14 (17) advances, which can result inperformance deterioration. Since the gas load F is large especially onthe high-pressure compressor unit, it is necessary to reduce the surfaceabrasion of the lip ring 9, the piston ring 10, and the cylinder 17 onthe high-pressure compressor unit to prevent the performancedeterioration.

The horizontal component f of the gas load F increases as the tilt angleθ corresponding to the angle at which the piston 4 (8) is tilted withrespect to the cylinder 14 (17) increases. In the embodiment of thepresent invention, as described above, the compressor is designed so asto prevent the high-pressure maximum tilt angle θ2 from becoming largerthan the low-pressure maximum tilt angle θ1. Therefore, especially onthe high-pressure compressor unit in which the surface abrasions of thelip ring 9, the piston ring 10, and the cylinder 17 could be a problem,it is possible to reduce the abrasions thereof and prevent theperformance deterioration.

As described above, in the embodiment of the present invention, thelow-pressure piston 4 and the high-pressure piston 5 are constructedaccording to the above-described dimensional relationship therebetween,thereby minimizing the tilt angle θ. Also, the low- and high-pressurebore diameters D and strokes S (the stroke S equals two multiplied bythe eccentric amount r), and connecting rod lengths l are determined bythe foregoing equations (1) to (14) so as to prevent the high-pressuremaximum tilt angle θ2 from becoming larger than the low-pressure maximumtilt angle θ1, thereby allowing prevention of the performancedeterioration due to leakage of compressed air, especially on thehigh-pressure side.

It should be understood that the foregoing description is onlyillustrative of the preferred embodiment of the present invention, andshould not be taken as a limitation of the technical scope of theinvention. That is to say, various embodiments of the invention arepossible without departing from the technical principles or essential ofthe invention.

1. A reciprocating compressor comprising: a low-pressure compressor unithaving a low-pressure piston and a low-pressure cylinder for compressingair with the low-pressure piston reciprocating while oscillating withinthe low-pressure cylinder; a high-pressure compressor unit having ahigh-pressure piston and a high-pressure cylinder for furthercompressing the air compressed in the low-pressure compressor unit withthe high-pressure piston reciprocating while oscillating within thehigh-pressure cylinder; and a motor for driving the low-pressurecompressor unit and the high-pressure compressor unit, wherein a maximumtilt angle during oscillation of the high-pressure piston is preventedfrom becoming larger than a maximum tilt angle during oscillation of thelow-pressure piston.
 2. The reciprocating compressor according to claim1, wherein a maximum gap formed between the high-pressure piston and thehigh-pressure cylinder during oscillation of the high-pressure piston isprevented from becoming larger than a maximum gap formed between thelow-pressure piston and the low-pressure cylinder during oscillation ofthe low-pressure piston.
 3. The reciprocating compressor according toclaim 1, wherein the low-pressure piston and the high-pressure pistoneach include: an eccentric portion coupled to a rotating shaft of themotor for performing rotational motion; a piston body for compressingair within the cylinder; and a connecting rod for connecting theeccentric portion and the piston body, the piston body being fixed tothe connecting rod.
 4. The reciprocating compressor according to claim3, wherein the piston body performs oscillating motion with therotational motion of the eccentric portion.
 5. The reciprocatingcompressor according to claim 1, wherein a bore diameter of thehigh-pressure piston is prevented from being made larger than a borediameter of the low-pressure piston.
 6. The reciprocating compressoraccording to claim 3, wherein a length from a center of the eccentricportion to a leading end of the low-pressure piston and a length from acenter of the eccentric portion to a leading end of the high-pressurepiston are made subequal.
 7. The reciprocating compressor according toclaim 1, wherein the low-pressure piston and the high-pressure pistoneach include a piston body for compressing air within the cylinder, andwherein the piston body of the low-pressure piston is provided with alip ring, and the piston body of the high-pressure piston is providedwith a piston ring.
 8. The reciprocating compressor according to claim1, wherein the low-pressure piston and the high-pressure piston eachinclude a piston body for compressing air within the cylinder, andwherein the piston body of the low-pressure piston is provided with apiston ring, and the piston body of the high-pressure piston is providedwith a piston ring.
 9. A reciprocating compressor comprising: a motorhaving a rotating shaft; a low-pressure compressor unit having alow-pressure cylinder and a low-pressure piston for compressing air; anda high-pressure compressor unit having a high-pressure cylinder and ahigh-pressure piston for further compressing the air compressed in thelow-pressure compressor unit, the low-pressure piston and thehigh-pressure piston each including: an eccentric portion for performingeccentric motion with rotation of the rotating shaft of the motor; aconnecting rod extending from the eccentric portion; and a piston bodyprovided on a leading end of the connecting rod, wherein, when aneccentric amount of the eccentric portion of the low-pressure pistonwith respect to the rotating shaft of motor is represented by r1 and aneccentric amount of the eccentric portion of the high-pressure pistonwith respect to the rotating shaft of motor is represented by r2, thelow- and high-pressure pistons are formed in such a manner that r1>r2 isestablished.
 10. The reciprocating compressor according to claim 9,wherein a maximum gap formed between the high-pressure piston and thehigh-pressure cylinder during oscillation of the high-pressure piston ismade smaller than a maximum gap formed between the low-pressure pistonand the low-pressure cylinder during oscillation of the low-pressurepiston.
 11. The reciprocating compressor according to claim 9, whereinthe low-pressure piston and the high-pressure piston each include thepiston body fixed to the connecting rod.
 12. The reciprocatingcompressor according to claim 9, wherein the respective piston bodiesreciprocate while oscillating within the high- and low-pressurecylinders with eccentric motion of the eccentric portion.
 13. Thereciprocating compressor according to claim 9, wherein a bore diameterof the high-pressure piston is made smaller than a bore diameter of thelow-pressure piston.
 14. The reciprocating compressor according to claim9, wherein, when a length from a center of the eccentric portion of thelow-pressure piston to a leading end of the piston body of thelow-pressure piston is represented by l1 and a length from a center ofthe eccentric portion of the high-pressure piston to a leading end ofthe piston body of the high-pressure piston is represented by l2, thelow- and high-pressure pistons are formed so that r1/l1>r2/l2 isestablished.
 15. The reciprocating compressor according to claim 9,wherein the piston body of the low-pressure piston is provided with alip ring, and the piston body of the high-pressure piston is providedwith a piston ring.
 16. The reciprocating compressor according to claim9, wherein the piston body of the low-pressure piston is provided with apiston ring, and the piston body of the high-pressure piston is providedwith a piston ring.
 17. The reciprocating compressor according to claim2, wherein the low-pressure piston and the high-pressure piston eachinclude: an eccentric portion coupled to a rotating shaft of the motorfor performing rotational motion; a piston body for compressing airwithin the cylinder; and a connecting rod for connecting the eccentricportion and the piston body, the piston body being fixed to theconnecting rod.
 18. The reciprocating compressor according to claim 10,wherein the low-pressure piston and the high-pressure piston eachinclude the piston body fixed to the connecting rod.